Go for the linen suit! Yes, evening weddings are more formal, but middle-of-the-summer weddings are less so (especially in Miami!). I was in the Catskills this past weekend for a wedding, much of which was either outdoors or in non-A/C'd or poorly A/C'd indoor rooms, and it was about 100*F out. Dark suit? No way!!! Even my tan silk suit was too warm for that nonsense! And to think the sister of the bride was trying to get people to tux it at the last minute!